Pertec Tape Drive Interface Musings

I was looking at a couple of documents describing the Pertec tape interface; the manual for my Kennedy 9610 tape drive, and a nice reference by a fellow with a rather familiar name: http://www.sydex.com/pertec.html According to my Kennedy manual, issuing a read command causes the drive to return one block of data. I can see how that would be used in block-oriented applications in which blocks may be randomly read, written and re-written on the tape. But most of my magtape experience has been using the tapes in a streaming mode, such as when reading/writing one or more tar archives separated by file marks. When writing a tar archive on a magtape from a Unix system, is the archive written as a sequence of fixed-size blocks? Or is the entire tar archive effectively written as one continuous block which must be streamed with no repositioning? I'm curious because I'm daydreaming about how to build a tape drive interface controller, and I wonder whether it might need to potentially stream an entire tape in one go vs. being able to safely assume some maximal block size.